The present invention relates to the sterilization and disinfection arts. It finds particular application in conjunction with the detection of peroxyacetic acid concentrations in solutions used for sterilization or disinfection of medical, dental, and pharmaceutical equipment and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to detection of peroxyacetic acid and other oxidizable chemicals in solution, such as hydrogen peroxide.
Peroxyacetic acid, or peracetic acid, is a useful sterilant and/or disinfectant for a variety of applications, including disinfection of waste and sterilization or disinfection of medical, dental, pharmaceutical, or mortuary equipment, packaging containers, food processing equipment, and the like. It has a broad spectrum of activity against microorganisms, and is effective even at low temperatures. It poses few disposal problems because it decomposes to compounds which are readily degraded in sewage treatment plants. Peroxyacetic acid solutions also have the ability to be reused over a period of time, allowing instruments to be sterilized or disinfected throughout the day in the same bath of sterilant.
In use, peroxyacetic acid precursors are typically mixed with water and other chemicals in order to create a sterilant solution. Items to be sterilized or disinfected are then immersed in the sterilant. Decontaminated items are then rinsed to remove traces of the acid and other cleaning chemicals, before use. To ensure effective sterilization or disinfection within a preselected period of time, the concentration of peroxyacetic acid is maintained above a selected minimum effective level. Disinfection is typically carried out at lower concentrations of peroxyacetic acid than for sterilization. When the peroxyacetic acid concentration is at or above the minimum effective level, complete sterilization or disinfection is expected.
Because the peroxyacetic acid tends to decompose over time, it is valuable to monitor the sterilant periodically to determine the level of peroxyacetic acid. The level can be compared against preselected minimum levels, used to adjust contact time, used to control concentration, or the like. Currently, it is often assumed that the sterilant will remain at or above the minimum effective concentration. However, differences in the temperature of the sterilant, the quantity of items sterilized or disinfected, and the degree and nature of contamination of the items all result in considerable variations in the degradation of the sterilant. In addition, storage conditions and duration sometimes lead to degradation of the peroxyacetic acid precursors before use.
Methods currently used to detect peroxyacetic acid are often unable to distinguish between peroxyacetic acid and other compounds typically present in the solution, such as hydrogen peroxide and acetic acid. Dippable papers are easy to use, but lack accuracy, particularly at concentrations suitable for sterilization or disinfection. Chemical titration methods provide a more accurate measure of the peroxyacetic acid in solution, but are time consuming to perform and are not readily automated. Frequently, more than one titration is performed to eliminate the contribution of hydrogen peroxide to the result.
Recently, a number of electrochemical techniques have been developed for detection of oxidizable or reducible chemical species, such as peroxyacetic acid. Consentino, et al., U.S. Pat. No. 5,400,818, discloses a sensor for peroxyacetic acid-hydrogen peroxide solutions. The sensor measures the resistivity of the solution, which is dependent on both the peroxyacetic acid and the hydrogen peroxide concentrations, as well as other factors. Thus, the sensor is unable to differentiate between the two components.
European Patent Application EP 0 333 246 A, to Unilever PLC, discloses an electrochemical sensor for detection of oxidizable or reducible chemical species using an amperometric method in which a fixed potential is maintained between a reference and a working electrode. The current at the working electrode is used to determine the concentration of peroxyacetic acid. Other species present, however, influence the current flowing, and hence the accuracy of the results.
Teske, U.S. Pat. No. 5,503,720, discloses a process for the determination of reducible or oxidizable substances, such as peroxyacetic acid in sewage waste. The process uses potentiostatic amperometry to detect peroxyacetic acid concentrations. The technique, however, depends on the achievement of a steady state, which frequently takes several hours.
Conventional electrochemical detection systems often employ a porous membrane, which separates the sample to be analyzed from the electrodes. Charged species pass through the membrane when traveling to the electrodes. This increases the time for measurements to be made and adds complexity and cost to the system.
The present invention provides a new and improved sensor and method for the selective detection of peroxyacetic acid which overcomes the above referenced problems and others.
In accordance with one aspect of the present invention, a decontamination process is provided. The process includes circulating a decontaminant solution including peroxyacetic acid though a treatment vessel which contains items to be decontaminated. The process further includes withdrawing a sample of the decontaminant solution into a chamber to contact a working electrode and a counter electrode and pulsing a voltage between the working electrode and the counter electrode at a selected voltage relative to a reference electrode and measuring the output current generated. The voltage is selected such that the current generated is substantially dependent on a concentration of the peroxyacetic acid in the sample and substantially independent of a concentration of another oxidizing species in the sample.
In accordance with another aspect of the present invention, a method of detecting a first oxidizing species in a solution to be tested is provided. The solution also contains a second oxidizing species. The method includes disposing a working electrode and a counter electrode in the solution, pulsing a read voltage in the diffusion limiting range across the working electrode and the counter electrode, and detecting current flowing between the working electrode and the counter electrode. The read voltage is selected such that the current flowing is substantially dependent on the concentration of the first oxidizing species and substantially independent of the concentration of the second oxidizing species in the solution.
In accordance with another aspect of the present invention, a decontamination apparatus is provided. The apparatus includes a decontamination vessel which receives items to be decontaminated. A fluid flow path circulates a decontaminant ion solution through the vessel. A sensor system is fluidly connected with the fluid flow path for specifically detecting the decontaminant in the decontaminant solution. The system includes a chamber which receives a sample of the decontaminant solution from the fluid flow path, a working electrode, and a counter electrode disposed within the chamber to contact the sample of decontaminant solution. An amperometric controller is electrically connected with the working and counter electrodes. The controller selectively pulses a preselected read voltage between the working electrode and the counter electrode and detects an output current flowing in a circuit including the working electrode, the counter electrode, and the solution. The read voltage is selected such that the output current is substantially dependent on the decontaminant concentration and substantially independent of the concentration of another oxidizing species in the solution.
One advantage of the present invention is that it enables the peroxyacetic acid concentration of a sterilizing or disinfecting solution to be determined rapidly, (i.e., in less than one minute) and without interference by other oxidizing species present in the solution.
Another advantage of the present invention is that the sensor confirms that a minimum effective concentration of peroxyacetic acid is maintained for effective sterilization or disinfection.
Yet another advantage of the present invention is the provision of a disposable sensor probe that requires no calibration before use.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.